The invention relates to a cold workable contact paste which contains carbonizable binding agents and a powdery solid component.
The use of carbon and graphite items e.g. in metallurgical furnace linings or the electrodes in cells for the electrolytic production of aluminum often leads to the problem of joining individual components or parts to make a unit which can also withstand elevated temperatures, and this such that the joints exhibit good thermal and electrical conductivity. Up to now it has only been possible to solve this problem satisfactorily in special cases. As carbon and graphite can, in general, not be welded, the joints are made using a contact paste which contains carbonizable binding agents and which, after being introduced into a suitably sized gap between the bodies to be joined, is transformed to carbon by heating and acts then as a binder. To be able to introduce the paste into the gaps, the paste must possess sufficient plasticity i.e. the mass, containing a coal tar pitch as a binder or petroleum pitch, must be heated to a temperature of about 100.degree. C. above its softening point before it is introduced into the gaps. Special devices are needed for this e.g. mixers which can be heated and which are fitted with facilities for measuring and controlling the temperature of the paste. Without these relatively expensive measures it is almost impossible to prevent damage occuring to the paste due to local overheating or the like. The viscosity of the contact paste increases with decreasing temperature. This means that relatively high working temperatures are necessary, at which even larger amounts of tars and other substances are emitted which could be harmful to the human body.
In practice it is not always possible to achieve the necessary requirements with the result that the quality of the joint may not match up to expectations. Contact pastes containing a pitch binder exhibit another property which limits their application. Only after reaching a temperature above .about.500.degree. C.--after coking has been completed--does the junction between the parts to be joined become mechanically strong and exhibits relatively good electrical and thermal conductivities. Cold workable contact pastes do not exhibit the above disadvantages and therefore numerous pastes of widely differing composition have been proposed with this in mind. Cold workable contact pastes generally contain resins which harden on heating or a special catalyst which accelerates the process of hardening or curing a resin at room tempeature. By adding solvents or diluting agents the plasticity of these pastes can be adjusted to a certain extent to suit the application. On the other hand contact pastes containing resins have only limited storage times if they are not stored at low temperatures, e.g. in cold rooms.
All contact pastes contain, besides the binding agent, a so-called solid component which minimizes the shrinkage of the binder during the coking process and is intended to improve the thermal and electrical conductivity of the green mass and of the coked contact paste. Preferred solid components are types of carbon and graphite, the chemical and thermal properties of which correspond to those of the carbon and graphite bodies to be joined e.g. anthracite, cokes, carbon black and graphite. Also suitable are refractory ceramic materials such as dolomite, corundum, periclase and the like. In general, under the term solids is understood materials which basically do not change when used in the contact paste. The amounts and particle size of the solids used depend among other things on the width of the gap at the joint, whereby in general the amount and particle size increase with increasing size of gap. Appropriate use of the contact pastes allows a strength of joint to be achieved which is in general adequate for the particular application. On the other hand the thermal and electrical conductivity of the joint is in many cases inadequate. To overcome this inadequacy a number of suggestions have been made including the proposal to add metal powders or turnings to the paste inasmuch as the temperature reached in the application in question would allow. Examples of this are presented in patents DE-AS No. 1,236,392, GB-PS No. 883,676 and US-PS No. 4,001,104.
A significant disadvantage in connection with the known contact pastes is that the properties which can be achieved are substantially determined by the conditions of manufacture including the rate of heating to temperature and the time at temperature, whereby the parameters related to the heating are given by the particular application. In general, therefore, for different applications use is also made of different pastes which are prepared with particular heating conditions in mind. For example the German Pat. No. 1 558 744 describes a contact paste for joining a metallic conductor and a carbon anode for an aluminum reduction cell, where the paste contains furfurylalcohol resin and graphite particles impregnated with an acidic catalyst. A contact paste for joining a metallic conductor and carbon cathode for the same electrolytic process is made up, according to the British Pat. No. 883, 676, of a mixture containing graphite intercalation compound graphite or metal powder and phenolic and furan resins. A contact paste for creating a connection between the individual carbon cathodes for the same process comprises, according to the German Pat. No. 25 09 550, essentially a mixture of anthracite, pitch and methyl naphthalene. There are therefore three different advantageous contact pastes for functionally related parts of a reduction cell.
A number of different contact pastes is also used for the linings of furnaces e.g. for blast furnaces used to produce iron, for tap hole plugs and the like. It is therefore an object of the present invention to develop a contact paste which has a wider range of application, is much less sensitive to scatter and changes in the processing conditions than the known pastes and can be produced at lower expense. A second object of the invention is that the paste exhibits a high degree of plasticity at room temperature and, even before the carbonizing of the binding agents, exhibits high thermal and electrical conductivity.